The present invention relates to power transmission systems, such as the drive train of a vehicle, having at least two outputs from a single input. More particularly, the invention concerns a differential arrangement between the input and the outputs.
In a typical drive train, such as for a vehicle, the output shaft from the power source (i.e., engine or motor) is coupled to two drive axles through a differential. Each drive axle provides power to a separate prime mover, such as a vehicle wheel or propeller. The differential was developed to perform two primary functions, namely to transmit power from a single source to two drive axles and to permit independent rotation of the two driven axles. This latter function accounts for the differential rotational speeds of the two drive axles that may occur as a result of variations in wheel rolling radii and/or during a turn. Of course, it is known that as a wheeled vehicle turns, the outboard wheel must travel farther, and therefore faster, than the inboard wheel. The differential accounts for this velocity difference by diverting more power to the outboard wheel, thereby driving that wheel faster than the inboard wheel.
In the course of development of automotive differentials, the need for traction management was recognized. One difficulty with a conventional open differential is that it is not capable of delivering additional torque to a non-slipping wheel when one drive wheel loses traction. This problem has been addressed by modified differentials, such as the limited slip differential and the Torsen differential. The typical limited slip differential utilizes friction clutches to oppose the transfer of torque between drive axles. The Torsen differential provides torque proportioning characteristics between drive axles. The Torsen differential utilizes an Invex gearing configuration to control the generation of frictional torques so that the torque differential is proportional to the torque provided to the drive train.
These prior differentials are reactive devices, meaning that they react to changes in the speed and torque needs of the driven axles. There is a desire in certain applications to positively control the difference in rotational speeds between the two output shafts or axles. One such application is to provide steering in a multi-propeller drive system such as for a boat or an airplane. In this application, a modified differential system allows for increased or decreased rotation of each output shaft. An example of one such modified differential system is found in U.S. Pat. No. 6,554,729 to Gleasman et al. In this system, each output shaft is coupled to the input through an orbital gear set. Steering control is provided through a worm gear arrangement that controls the orbital rotation of the gear set. When the worm gear arrangement prevents orbital rotation, power is transmitted from the input shaft to the output shaft only by the rotation of each cluster gear within the orbital gear set. Rotation of the worm gear controls the orbital rotation of the cluster gears to either add to or subtract from the output shaft.
One disadvantage of the system shown in the '729 Patent and in other similar systems is that gearing arrangement is difficult to package, or more particularly that the envelope required to house the system is too large for many applications that might benefit by the controllable differential rotation characteristics of the system. Another disadvantage is that these systems are mechanically inefficient due to gear backlash, contact problems between worm gears and the like. There is a need for a differential-based steering system that addresses these disadvantages.